The present invention relates to ball thread devices, and more particularly to an improvement of a ball-circulating-tube fixing structure in an external circulation type ball thread device.
An external circulation type ball thread device is available in which, as shown in FIG. 8, a ball nut 2 is engaged through a number of balls (not shown) with a threaded shaft 1, and a ball circulating tube 4 is provided on the outer surface of the ball nut 2 so that the balls are circulated while being led outside the ball nut 2. In the ball thread device, the ball circulating tube 4 is fixedly mounted on the outer surface of the ball nut 2 with a tube retaining member 5. The tube retaining member 5 is formed as follows: As shown in FIGS. 10 and 11, a metal block whose thickness is larger than the outside diameter of the ball circulating tube 4 is formed into a rectangular parallelepiped by milling. A tube engaging groove 6 is formed in the rectangular parallelepiped by end milling in such a manner that it is extended obliquely in the bottom. Thereafter, a plurality of flat countersunk head screw holes 7 (two holes in the case of FIG. 10) are formed in the rectangular parallelepiped on both sides of the tube engaging groove 6 by drilling. Thus, the tube retaining member 5 has been fabricated.
On the other hand, as shown in FIG. 9, the outer surface of the ball nut 2 has a flat surface 8 which is formed by milling to mount the ball circulating tube 4 and the tube retaining member 5 on it. Holes 9 for mounting the ball circulating tube 4 are formed in the flat surface 8, and in addition threaded holes 10 for mounting the tube retaining member 5 are formed in the flat surface 8 by tapping.
However, the above-described the external circulation type ball thread device suffers from the following problems:
As was described above, in the conventional device, the tube retaining member 5 is formed through a number of manufacturing steps including milling, end milling and drilling steps, and in order to form the flat surface 8 on the ball nut 2, a relatively large part of the nut is removed by milling, and the threaded holes for mounting the ball retaining member 5 are formed by tapping. That is, the formation of a ball-circulating-tube mounting structure takes a lot of time and labor.
On the other hand, the ball circulating tube 4 is mounted on the ball nut as follows: Both end portions of the ball circulating tube 4 are inserted into the mounting holes 9 and 9 formed in the ball nut 2, respectively, and the ball circulating tube 4 is held with the tube retaining member 5, and small flat countersunk head screws B.sub.1 are engaged with the threaded holes 10 and tightened. Thereafter, in order to eliminate the play of the ball circulating tube which is due to the gap C (FIGS. 11 and 12) between the ball circulating tube 4 and the tube engaging groove 6 formed in the tube retaining member 5, the retaining member must be adjusted as follows: With the screws B.sub.1 loosened, the tube retaining member 5 is removed from the ball nut 2. And the bottom of the tube retaining member 5 is ground until the gap C is substantially zero. That is, assembling the device takes a lot of time and labor.